Isotope-Based Techniques to Investigate Factors Influencing Water Use Efficiency in Pinus koraiensis Leaves during Plant Growth

Plant water use efficiency (WUE) is a comprehensive physiological indicator of plant growth and ability to adapt to drought. However, research on the mechanisms controlling WUE during plant growth and development remains weak. Here, we studied Pinus koraiensis as a typical evergreen conifer species in Northeast China. After collecting 80 tree samples with varying diameters at breast height (DBH), we measured &delta:13C and &delta:18O as an indicator of WUE, leaf morphology (volume, dry weight, and total epidermal area), ecological stoichiometry (carbon, nitrogen, and phosphorus content), and abiotic factors (light environment, soil pH, soil water content, and soil nutrient content). Correlational analysis of these variables revealed distinct differences between smaller/younger and larger/older plants: (1) In plants with DBH less than 52 cm, &delta:13C was positively related to DBH, and &delta:18O was negatively related to DBH. Plants with DBH greater than 52 cm showed no relationship between &delta:13C and DBH, and &delta:18O was positively related to DBH. (2) In plants with DBH less than 52 cm, there was a negative correlation between &delta:13C and &delta:18O and between &delta:13C and leaf phosphorus content (LP), but a positive correlation between &delta:13C and DBH, leaf mass per area (LMA), and leaf density (LD). The slopes of DBH-&delta:13C, &delta:18O-&delta:13C, leaf nitrogen content (LN)-&delta:13C, and LMA-&delta:13C correlations were greater in smaller plants than large plants. (3) Structural equation modelling showed that in smaller plants, DBH had a direct positive effect on &delta:13C content and a direct negative effect on &delta:18O, and there was a direct positive effect of light environment on &delta:18O. In larger plants, there was a direct negative effect of light environment on &delta:13C and a direct positive effect of DBH on light environment, as well as a negative effect of soil nitrogen content on leaf nitrogen. In smaller plants, DBH was the most important factor influencing &delta:13C, followed by &delta:18O and soil moisture, with light and soil pH showing minimal influence. In larger plants, light environment influenced &delta:13C the most, followed by soil nitrogen content and soil moisture content, with leaf nitrogen and DBH contributing little. The results suggest that water use efficiency strategies of P. koraiensis vary according to growth stage, and the effects of abiotic factors and functional traits vary at different growth stages.